(A) Electrode Foils and Methods of Manufacture Thereof
Aluminum electrode foils for electrolytic capacitors are generally produced using as the starting material at least 98% high-purity aluminum that has been rendered into the form of a foil by rolling. Innumerable pits are formed on the surface of the aluminum foil using a known direct-current electrolytic etching process, an alternating-current electrolytic etching process, or both in alternation, within an acid or alkali solution that dissolves aluminum, thereby enlarging the actual surface area and increasing the electrostatic capacitance.
With such etching methods, a multiplicative effect per unit surface area of several hundred-fold, generally about 300 to 400 times, is achieved.
An advantage of electrolytic capacitors is that, because the dielectric layer is composed of an extremely thin oxide film layer, the electrostatic capacitance per unit surface area of the electrode is high, enabling electrolytic capacitors which are small yet of large capacitance to be obtained. However, with the miniaturization of electronic devices, there exists a need for electrolytic capacitors of even smaller size and higher capacitance.
When attempting to increase the capacitance per unit surface area by etching treatment as described above, the entire aluminum foil becomes filled with etching pits, which makes the electrode foil fragile and lowers the electrode foil strength.
To address this problem, one method for achieving a large electrode foil surface area while maintaining the foil strength is to etch an aluminum foil of substantial thickness and thereby increase the capacitance per unit surface area.
In this method, the capacitance per unit surface area is large, but when such an electrode foil is used to construct a common coiled electrolytic capacitor, the large thickness of the electrode foil places limits on the length of electrode foil that may be used, resulting in the formation of a capacitor element which will not fit within the case.
In addition, in the case of multilayer solid electrolytic capacitors, due to height constraints resulting from the shape, it is hard to make the height too large. In forming the capacitors from a foil with reduced strength such as that mentioned above, it is hard to set the welding conditions for stacking, and problems with the strength after welding also arise.
In thus trying to achieve a higher capacitance, the number of stacked layers may be increased or thick electrode foils may be used, resulting in an increase in the height of the capacitor.
In addition to the general prior art mentioned above, in the case of coiled electrolytic capacitors, as described in Patent Document 1, a method is known which, based on the fact that the capacitance is computed as the composite capacitance, increases the capacitance by forming a vapor-deposited film of metal nitride on a substrate surface. Also, Patent Document 2 discloses a method for achieving an increased capacitance by using foil which has been brought into physical contact with carbon as the cathode to cancel the cathode-side capacitance so that only the anode-side capacitance is reflected in the composite capacitance.
(B) Electrolytic Capacitors
Electrolytic capacitors are electronic components which are widely used in electronic devices, and generally have the following type of construction.
The action of having resistance to a voltage applied in one direction but losing such resistance when a voltage is applied in the opposite direction is called a “valve action.” An electrolytic capacitor uses a valve metal (e.g., aluminum) having such a valve action as the anode. By means of treatment such as anodic oxidation, an insulating oxide film is formed on the anode surface.
This oxide film acts as a dielectric layer; an electrolyte such as an electrolytic solution or an electrically conductive polymer, or a solid electrolyte, is in contact with the oxide film substantially as a cathode.
In a coiled capacitor, the electrolyte is held by electrolytic paper (separator) or the like. In a flat-plate capacitor, a carbon paste and metal particles with a resin material are formed on the electrolyte in this order, resulting in a cathode layer.
In each type of electrolytic capacitor, the cathode-side electrode is led out from a cathode made of a metal such as aluminum.
The anode-side electrode and the cathode-side electrode are generally made of foils that have been cut into strips. In a coiled capacitor, the capacitor element is formed by winding the foils together with a separator. In a flat-plate capacitor, the capacitor element is formed by arranging as an anode-side electrode, a cathode layer, and a cathode-side electrode or the like, all of which are of square shape. Stacking together a plurality of such capacitor elements results in a multilayer capacitor.
The cathode layer is connected to a lead-out electrode which is composed primarily of a metal and electrically connects the cathode layer to the exterior, and is thereby led out to the exterior from any of various types of housings which encase the capacitor element.
Here, because the dielectric layer is composed of an extremely thin oxide film layer, the electrolytic capacitor is characterized by a high electrostatic capacitance per unit surface area of the electrode, enabling a small capacitor of large capacitance to be obtained. However, with the miniaturization of electronic devices, smaller sizes and larger capacitances are being demanded even of electrolytic capacitors.
To increase the capacitance of electrolytic capacitors, enlargement of the surface area by using etching treatment or the like to roughen the electrode surface has hitherto been carried out. However, the surface area enlargement by such roughening has been approaching the limit recently, new measures for increasing the electrostatic capacitance are being sought.
For example, by etching an aluminum foil to form countless small pits extending deeply into the inner portion of the aluminum foil in order to increase the capacitance per unit surface area, a larger capacitance per unit surface area may be achieved.
However, when attempting to increase the capacitance per unit surface area in the above manner, the entire aluminum foil becomes filled with etching pits, which makes the electrode foil fragile and lowers the electrode foil strength.
To address this problem, one method for achieving a large electrode foil surface area while maintaining the foil strength is to etch an aluminum foil of substantial thickness and thereby increase the capacitance per unit surface area. In this method, the capacitance per unit surface area is large, but when such an electrode foil is used to construct common coiled electrolytic capacitors, the large thickness of the electrode foil places limits on the length of electrode foil that may be used, resulting in the formation of a capacitor element which will not fit within the case.
In addition, in the case of multilayer solid electrolytic capacitors, due to height constraints resulting from the shape, it is hard to make the height too large. In forming the capacitors from a foil with reduced strength such as that mentioned above, it is hard to set the welding conditions for stacking, and problems with the strength after welding also arise.
In thus trying to increase a higher capacitance, the number of stacked layers may be increased or thick electrode foils may be used, resulting in an increase in the height of the capacitor.
In addition to the general art mentioned above, in the case of coiled electrolytic capacitors, as described in Patent Document 1, a method is known which, based on the fact that the capacitance is computed as the composite capacitance, increases the capacitance by forming a vapor-deposited film of metal nitride on a substrate surface. Also, Patent Document 2 discloses a method for achieving an increased capacitance by using foil which has been brought into physical contact with carbon as the cathode to cancel the cathode-side capacitance so that only the anode-side capacitance is reflected in the composite capacitance.    Patent Document 1: Japanese Laid-open Patent Publication No. 02-117123    Patent Document 2: Japanese Patent No. 3,875,705